Pavel Rodin

Novel Closing Switches Based on Propagation of Fast Ionization Fronts in Semiconductors

New concept of triggering the fast ionization fronts in semiconductors with the field-enhanced ionization of deep electron traps is described. Closing switches designed on the base of this concept has been called the Deep Levels Dinistors, DLDs. These devises are able to form high current pulses with subnanosecond risetime and low voltage drop after switching. As an instance three generators based on DLDs are described. The possibility of picosecond switching on the base of tunnel assisted impact ionization front is discussed.

Field-enhanced ionization of deep-level centers as a triggering mechanism for superfast impact ionization fronts in Si structures

We investigate the origin of free carriers that initiate impact ionization in depleted high-voltage p-n junctions under dynamic breakdown conditions and deterministically trigger superfast ionization fronts that propagate several times faster than the saturated drift velocity. We argue that in Si structures triggering occurs due to the field-enhanced ionization of process-induced deep-level centers identified as sulfur impurities. This impurity is a double-level electron trap with low recombination activity. It is present in high-voltage Si structures due to the side effect of widely used fabrication technology. We calculate the field and temperature dependences of the ionization probability for the upper midgap level (0.28eV) and midgap level (0.54eV) in electric fields up to 5×105V∕cm as well as the occupation of these levels at different temperatures. The emission of free electrons is sufficient to trigger the ionization front from zero temperature to ∼400K, in agreement with experiments. At room tempe...

Breathing Current Domains in Globally Coupled Electrochemical Systems: A Comparison with a Semiconductor Model

Spatio-temporal bifurcations and complex dynamics in globally coupled intrinsically bistable electrochemical systems with an S-shaped current-voltage characteristic under galvanostatic control are studied theoretically on a one-dimensional domain. The results are compared with the dynamics and the bifurcation scenarios occurring in a closely related model which describes pattern formation in semiconductors. Under galvanostatic control both systems are unstable with respect to the formation of stationary large amplitude current domains. The current domains as well as the homogeneous steady state exhibit oscillatory instabilities for slow dynamics of the potential drop across the double layer, or across the semiconductor device, respectively. The interplay of the different instabilities leads to complex spatio-temporal behavior. We find breathing current domains and chaotic spatio-temporal dynamics in the electrochemical system. Comparing these findings with the results obtained earlier for the semiconductor system, we outline bifurcation scenarios leading to complex dynamics in globally coupled bistable systems with subcritical spatial bifurcations.

Dynamic avalanche breakdown of a p‐n junction: Deterministic triggering of a plane streamer front

We discuss the dynamic impact ionization breakdown of a high voltage p‐n junction which occurs when the electric field is increased above the threshold of avalanche impact ionization on a time scale smaller than the inverse thermogeneration rate. The avalanche-to-streamer transition characterized by generation of dense electron-hole plasma capable of screening the applied external electric field occurs in such regimes. We argue that the experimentally observed deterministic triggering of the plane streamer front at the electric-field strength above the threshold of avalanche impact ionization, yet below the threshold of band-to-band tunneling, is generally caused by field-enhanced ionization of deep-level centers. We suggest that the process-induced sulfur centers and native defects such as EL2, HB2, and HB5 centers initiate the front in Si and GaAs structures, respectively. In deep-level-free structures the plane streamer front is triggered by Zener band-to-band tunneling.

Theory of superfast fronts of impact ionization in semiconductor structures

We present an analytical theory for impact ionization fronts in reversely biased p+-n-n+ structures. The front propagates into a depleted n base with a velocity that exceeds the saturated drift velocity. The front passage generates a dense electron-hole plasma and in this way switches the structure from low to high conductivity. For a planar front we determine the concentration of the generated plasma, the maximum electric field, the front width, and the voltage over the n base as functions of front velocity and doping of the n base. The theory takes into account that drift velocities and impact ionization coefficients differ between electrons and holes, and it makes quantitative predictions for any semiconductor material possible.

Theory of traveling filaments in bistable semiconductor structures

We present a generic nonlinear model for current filamentation in semiconductor structures with S-shaped current-voltage characteristics. The model accounts for the Joule self-heating of a current density filament. It is shown that the self-heating leads to a bifurcation from static to traveling filaments. Filaments start to travel when an increase of the lattice temperature has a negative impact on the cathode-anode transport. Since the impact ionization rate decreases with temperature, this occurs for a wide class of semiconductor systems whose bistability is due to the avalanche impact ionization. We develop an analytical theory of traveling filaments which reveals the mechanism of filament motion, finds the condition for bifurcation to traveling filament, and determines the filament velocity.

Anomalous dynamics of the residual voltage across a gallium-arsenide diode upon subnanosecond avalanche switching

A qualitative difference between high-voltage gallium-arsenide diodes and similar silicon devices is found experimentally upon ultrafast switching in the delayed avalanche breakdown regime. It is shown that, following switching, a gallium-arsenide diode remains in a highly conductive state throughout the entire duration of the applied voltage pulse and the recovery of the reverse voltage across the p-n junction due to the dispersal of nonequilibrium electron-hole plasma is not observed. In the same interval of time (2 ns in our experiment), a silicon diode passes completely into a blocking state. The residual voltage amplitude for a gallium-arsenide diode is an order of magnitude lower than that for a silicon device. The discovered effect is similar to a known effect of “sticking” of gallium-arsenide diode switches (the lock-on effect), which are triggered by a laser pulse, in a conductive state.

Subnanosecond avalanche switching in high-voltage silicon diodes with abrupt and graded p - n junctions

The phenomenon of delayed avalanche breakdown in high-voltage silicon diodes has been studied for the first time using an experimental setup with specially designed resistive coupler as a part of a high-quality matched measuring tract. Three types of diode structures with identical geometric parameters and close stationary breakdown voltages within 1.1–1.3 kV have been studied, including p+-n-n+ structures with abrupt p-n junctions and two different p+-p-n-n+ structures with graded p-n junctions. Upon switching of all structures, a voltage step with an amplitude above 1 kV and a rise time of ∼100 ps at a breakdown voltage of about 2 kV is formed in the load. However, switching to a state with low (∼150 V) residual voltage has been observed only in the structures with an abrupt p-n junction, while the voltage in structures with graded junctions only decreased to a level of ∼1 kV, which is close to the stationary breakdown voltage.

Numerical simulation of spatially nonuniform switching in silicon avalanche sharpening diodes

The process of spatially nonuniform switching in high-voltage silicon diodes operating in the delayed avalanche regime has been numerically simulated. The dependence of the transient process on the ratio between the total diode cross-section area and the area of the region where the switching takes place has been studied. The switching time (60–70 ps) and qualitative form of the transient characteristic agree with the available experimental data. It is established that a rapid drop of the diode voltage begins after the ionization front has traveled over most of the base and then continues due to secondary avalanche breakdown of the base filled with free carriers. Thus the time of switching to the conducting state exhibits no direct correlation with the velocity of ionization front propagation.

Lateral current density fronts in asymmetric double-barrier resonant-tunneling structures

We present a theoretical analysis and numerical simulations of lateral current density fronts in bistable resonant-tunneling diodes with Z-shaped current–voltage characteristics. The bistability is due to the charge accumulation in the quantum well of the double-barrier structure. We focus on asymmetric structures in the regime of sequential incoherent tunneling and study the dependence of the bistability range, the front velocity, and the front width on the structure parameters. We propose a sectional design of a structure that is suitable for experimental observation of front propagation and discuss potential problems of such measurements in view of our theoretical findings. We point out the possibility to use sectional resonant-tunneling structures as controllable three-terminal switches.